Polymer and glass fibers are used in a variety of fibrous materials, including non-woven fabrics, composite materials, filter media and building insulation. Fiber diameter significantly affects the properties of the fibrous material, including material strength, pore size and thermal resistivity. For this reason, it is important to ascertain the size (width or diameter) of fibers for process control purposes while manufacturing the fibrous material and/or for cataloging or grading the manufactured fibrous material for intended purpose.
One well-known technique for fabrication of non-woven fibrous material is a meltblown technique. Material is heated (melted) in a furnace and extruded through a plurality of orfii. The molten material is further attenuated by an air stream that blows along the direction of extrusion. The material cools in the air stream to form elongated fibers. The fibers are collected on a rotating collection drum as a non-woven web of fibers. Fiber size is affected by the viscosity of the molten fiber material exiting the orfii and the air pressure and temperature that draws the material from the furnace and cools it. In some manufacturing environments, it is desirable to measure the fiber size as the fibers are formed in the air stream, before collection on the drum. The fiber size data could be used to control the manufacturing process to regulate air pressure and temperature and furnace temperature, thereby controlling the size of fibers forming the web. In other environments, samples of the completed web are collected from the drum and examined to measure the size of fiber in the web. The web from which the sample was taken is thereby cataloged or graded.
Existing techniques for measuring fiber size include direct microscopic measurement (including electron microscopy) and indirect measurement based on airflow resistance of a compacted fiber sample. These methods are time-consuming and do not provide a continuous signal suitable for process control. More particularly, during fabrication of a fibrous web, such as by the meltblown process, the fibers move in a continuous manner through the process steps. Real-time measurement of the width of the fiber during the process would be best accomplished by a non-contact technique.
Non-contact techniques for size measurement include digital imaging, back-scattered non-imaging techniques, laser interferometric techniques, and forward-scattered non-imaging techniques. Digital imaging measures the size of the elongated object. Conveniently, the digital imaging can be accomplished in a backward direction, so that the light source and the imaging device are located in the same measuring probe. An example of digital imaging technique to measure size may be found in U.S. Pat. No. 4,887,155. One problem of digital imaging is that it requires the object being measured be within the focal region of the imaging optics. Consequently, the process is not well suited for measuring objects that are freely flowing, such as in a meltblown process. Further, the imaging process is best suited for large objects (several hundred microns wide) and is not altogether suitable for fine objects (a few microns wide).
The back-scattered non-imaging technique uses back-scattered light intensity for size measurement. This technique is capable of measuring small object widths, but requires the objects be retained in a fixed plane. Consequently, this process is not well suited for measuring freely flowing objects. Moreover, the signal generated by this technique is affected by the optical properties of the objects, so the process is not well suited for measuring objects of varying optical characteristics. Where a plurality of elongated objects is being measured, the technique is capable of measuring only the mean size of the objects, and not a size distribution. See U.S. Pat. No. 4,343,637 to Shofner et al.
Interferometric laser techniques for non-contact real-time measurement of the size of elongated objects are described in U.S. Pat. No. 3,953,128 (Holly), U.S. Pat. No. 5,355,209 (Grosso et al.), U.S. Pat. No. 5,432,605 (Naqwi et al.), U.S. Pat. No. 5,453,837 (Naqwi) and U.S. Pat. No. 5,513,004 (Naqwi). These techniques work well for isolated cylindrical fibers of homogeneous material, but are not well suited for measuring fibers whose internal structures are not homogeneous. Consequently, the laser interferometric technique is not suitable for measuring fibers produced by several important fiber-generating processes, including meltblown processes, which produce fibers that are not homogeneous in internal structure. Moreover, the orientation of the objects must be substantially uniform, so the process is not well suited for identifying fiber sizes in non-woven web material that consists of long, narrow entangled fibers.
Forward-scattering non-imaging techniques employ forward scattering of light for size measurement of elongated objects. One forward-scattering technique, described in U.S. Pat. No. 3,812,376 (Takeyama et al.), U.S. Pat. No. 4,009,965 (Pryor), U.S. Pat. No. 4,854,707 (Ring et al.) and U.S. Pat. No. 4,880,991 (Boehnlein et al.), scans the object being measured with a laser beam. This method requires the object be mounted at a fixed position, and is best suited for measuring large-sized objects, such as machine parts. Another approach, described in U.S. Pat. No. 4,280,827 (Murphy et al.), U.S. Pat. No. 4,390,897 (Smithgall et al.), U.S. Pat. No. 4,882,497 (Inoue et al.) and U.S. Pat. No. 5,015,867 (Siegel et al.) fully illuminates the elongated object, but is useful only for measuring single objects. This technique can not measure plural elongated objects to obtain size distributions of objects present in the measurement region.
A variant of the forward-scattering technique, is described by Kurokawa et al. in U.S. Pat. No. 6,459,494, and measures the blockage of a laser beam by an elongated object. Thus, the Kurokawa device measures the shadow cast by the elongated object. Although a plurality of the objects is illuminated by a single light source, each object requires a dedicated detector and must be positioned at a fixed location from the detector in order to measure its size. Another variant is described by Rochester in U.S. Pat. No. 5,264,909, and employs a lens to image the shadow of the elongated object onto a detector array. Like the Kurokawa device, the Rochester device requires the object be precisely in focus for measurement. Thus, the Kurokawa et al. and Rochester devices cannot measure sizes of randomly-located and randomly-oriented objects.
Blohm et al., in published U.S. Patent Application No. US2002/0044289, describes a hybrid of the shadow and diffraction measurement techniques by placing the object at a controlled distance from the detector. This technique is useful only for measuring sizes of single objects and does not allow random location or random orientation of the object.
Powers and Somerford, in “Fiber Sizing using Fraunhofer Diffraction” published in Optics Communication, 1978, infer a width distribution of a plurality of elongated objects (fibers) using a forward scattering technique. The objects being measured are positioned in a fixed plane so that the scattering pattern is invariant at a fixed distance from the objects. This approach can not measure randomly-located fibers. Moreover, the Powers and Somerford approach is limited to measurement of opaque elongated objects or elongated apertures in an opaque screen. The detector proposed by Powers and Somerford cannot perform real-time measurement and cannot provide comparable sensitivity of the measurement to objects with small and large widths.
There is a need for a practical measurement device that measures sizes of a plurality of elongated objects at random orientations and/or locations.